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From [zen_tom]'s comment in Cosmic Background Refrigeration. [link]
//And if so, in the vacuum of space, mightn't this qualify as a form of propulsion?//
Simply this: hot objects radiate photons in the near infra red. Out in space, in full view of the unshielded sun, objects get toasty hot,
and due to the surrounding vacuum, radiating photons is the only way of dumping that heat.
So form a black metal into a thin [parabola] correction: a sphere with a tiny hole at one point, or a hemisphere, or something like that. Float it in full sunlight, and spin it around its main axis so that it is evenly heated all over.
The inside surface is shaded from the sun, and can dump heat (infra-red light) only towards the cold inky blackness of deep space.
An evolutionary form consists of a long heatpipe in a parabolic trough, facing side-on to the sun. Shaded from the sun at the rear of the heatpipe is a black spherical radiator bulb, at the focus of a paraboloid bell (basically an infra-red torch). The bell points backwards along the axis of the craft. Perhaps it can be swivelled somewhat for steering.
The trough focuses sunlight on the length of the heatpipe, which transfers heat to the radiator. IR photons shoot out the back in a focused stream.
Yes, some photons will also shoot out the front and sides, but given the temperature differential between the sun-facing and space-facing ends/sides of the apparatus, the sum should still have a forward vector.
I have no idea how this compares to the solar sail effect of the incident light pressure, but perhaps the two forces can be used one against the other, as a way of tacking.
Cosmic Background Refrigeration
Cosmic_20Background_20Refrigeration
whence [BunsenHoneydew, Aug 03 2006]
IKECE
IKECE
[Vernon]'s radiator plus. [zen_tom, Aug 03 2006]
Crookes Radiometer
http://en.wikipedia.../Crookes_radiometer
"...the cause of much scientific debate..." [zen_tom, Aug 03 2006]
Solar Sails
http://en.wikipedia.org/wiki/Solar_sail
[zen_tom, Aug 03 2006]
Radiation Pressure
http://en.wikipedia.../Radiation_pressure
"...energy flux density divided by the speed of light..." [zen_tom, Aug 03 2006]
Nichols Radiometer
http://en.wikipedia.../Nichols_radiometer
This is the one that measures the pressure exerted by EM radiation - the Crookes version turns out to work due to subtle airflows etc [zen_tom, Aug 03 2006]
Momentum
http://en.wikipedia...ativistic_mechanics
Momentum of massless objects [monojohnny, Aug 03 2006]
[marked-for-engineering]
[BunsenHoneydew, Sep 27 2007]
Heatpipe
http://en.wikipedia.org/wiki/Heatpipe
[BunsenHoneydew, Sep 27 2007]
Black body radiation
http://en.wikipedia.org/wiki/Black_body
The sphere with a small hole is described as a perfect black body. It also mentions about a fifth of the way down next to the picture of the lava that its a perfect (theoretically of course) Lambertian Radiator. [bleh, Sep 27 2007]
Lambertian Radiator
http://en.wikipedia...bert%27s_cosine_law
An observer off the normal will observe the same intensity as an observer on the normal. [bleh, Sep 27 2007]
Yarkovsky effect
http://en.wikipedia...ki/Yarkovsky_effect
"... is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum. .... while tiny, could lead to large long-term effects in the orbits of small bodies ..." [BunsenHoneydew, Oct 17 2008]
YORP Effect
http://en.wikipedia...80%93Paddack_effect
combined anistropic radiation effects of rotation, shape and albedo [BunsenHoneydew, Apr 09 2010]
[link]
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//Yes, photons will also shoot out the front and sides, but because they radiate in all directions, the sum should still have a forward vector// |
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This is the part I find difficult to believe - I think the sum total of forces from radiation in all directions - despite shape - would be the same. |
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If indeed you can direct radiation emission by purely topographical means, then yes, this should be the result. |
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Perhaps you could cheat a little by coating the object with different materials front and back - but again, I'm not sure whether material A will radiate more than material B - when both materials are at the same temperature. |
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[Vernon] came up with an idea for an accelerated radiator - which might prove interesting in this context - however, once again - I believe that the sum total of all forces would still net to zero. |
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// Spin it so that it gets evenly heated all over // |
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// Point the pointy end where you want to go // |
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Is this a two-phase operation then ? |
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If so, it'll need some additional energy to stop it spinning. |
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mmmh. I'm well out of depth here....but isn't there a problem of momentum here... |
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How much momementum does a photon have ? Well not much, but we don't really care - as long as ejection of the photons in a prevalent direction. |
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But - the photons originally came from the sun.....when the photons are initially absorbed by this craft - will this alter the mass of the craft, so that it weighs more ? (E=mc2?) - and therefore negate the acceleration effect gained by the ejection of the photons ? (the force would be the same, but acting on a larger mass?) |
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[monojohny] I think we can ignore the effects of the weight of the absorbed photons from the sun - that's the process by which solar sails operate - and they (theoretically - I don't know if anyone's actually successfully tried it out) work, despite the mass increase delivered to them by their propelling photons. |
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The question here is whether IR radiation emitted by an object of some temperature can be directed into a beam based on an object's shape. |
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If we come up with a solution - we should be able to translate it to other radiative objects - allowing us, for example, to make a propulsion unit out of a block of cleverly shaped Uranium, Plutonium or some other radioactive substance. |
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[zen_tom] - I agree ; solar sails would seem to work despite this... |
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However, this craft is effectively turning photons around - firing them back at 180 degrees where they came from.....
(in a statistical manner, if not photon-by-photon)... |
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This is going to take some energy isn't it ?? |
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[zen_tom] - also good point about generalizing the system here.... but its also worthing annotating here that the radiation from Uranium etc is a different type of radiation - electrons and helium nuclei; in a sense that would be more like an ion-drive (I guess)...... |
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I'm not yet convinced about the shape being important here. |
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But as you said [zen_tom] different materials (or colours) front and back would definately work I think; because this system already exists... |
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Those little propellor things in an evacuated bulb with a black and a white side, which spin round when you put them in the sun. |
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I can't remember what they're called, but you get them in gadget shops and science museum shops etc. |
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//This is going to take some energy isn't it ??
//
The photon doesn't have to get bounced back at 180 degrees - say the object is orbiting the heat-source (a star probably) - the delivery of the energy could be at 90 degrees to the expulsion of the energy and resultant motion. i.e. from the side. |
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It's at times like these, I'd love to be able to draw a little diagram. |
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Those spinny things work on the same principles as the solar sail - which is a slightly different concept/process to the radiative propulsion. |
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180 degrees or 90 degrees... |
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I think the thing is going to have to extract energy here; I would *guess* that the absorbed radiation would be of a higher frequency that of the expelled photons.... |
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Er, just showing my ignorance here, but as photons have zero mass, how could they impart any momentum to the thingy? <gets ready to duck> |
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Check out wikipedia link which gives the formula for 'massless' particles....( I don't get it either - I guess something to do with the equivalence of mass and energy...) |
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Less efficent than a solar sail. |
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Lets say your system and a solar sail both interact with 100 photons. |
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The solar sail reflects all of the photons in an elastic collision. This imparts 2x the momentum of the photons to the craft. So if you call the momentum of one photon Pm, you have imparted 200Pm to your craft. |
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Your absorb and radiate craft gets those same 100 photons. When it absorbs their energy as heat, it is getting no momentum. Now it is going to reradiate that energy as thrust. Lets assume that your craft is cleverly designed to emit 75% of the heat out the back for thrust, 20% out the sides and 5% out the front. The 20% is a simple loss, the 5% opposes your desired action. So you are only getting 70Pm worth of momentum. |
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Basically you get three times the momentum from a solar sail. |
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I think what you want is an object to absorb radiant energy (ie a black object) connected to a good radiator (ie a metal or white object). |
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The black object must be a good conductor of heat or at least conductive... |
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//When it absorbs their energy as heat, it is getting no momentum// |
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So what you are saying is that you need a white object (a sail) to travel away from the sun and a black white object to travel towards it... And if so what is the difference between a black white object and a transparant one? |
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It appears that Galbinus_Caeli's analysis may be incorrect. |
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What is with all these parabolas? Here it adds nothing except to show that you don't understand optics. In any case, this idea is an old one, being the original (and mostly incorrect) explanation for the rotation of the vanes in the Crookes radiometer. Better to use the solar wind for your sail.
//as reaction mass//
Photons have momentum, not mass. |
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I may be completely wrong. Its not impossible. |
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But you can use a solar sail both to move away from the sun or towards it. Or more precisely to move into higher and lower orbits. Simply thrusting directly toward or away from the sun will do very little. You have to use the sail (or thermal thruster, or magic eightball flinging monkey, or whatever) to increase or decrease your orbital velocity. This raises or lowers your orbit. |
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Ah ! Radiometer ! That's what its called ! |
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What if you are not in orbit...? |
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Am kind of hoping you are not wrong actually --- seems like a nice idea (that is using the suns energy to travel towards it)... |
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If you are not in orbit, you are not going to get enough thrust to keep you from falling into the sun. |
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The parabolic geometry is all wrong of course - there being no central point source. I whipped this up quickly on my way out the door without really stopping to think about it. |
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Better would be a sphere or spheroid with a tiny aperture at one end (the classic "black body"). There still might be some thermodynamic reason why this won't produce a forward sum vector, to do with dynamic equilibrium of heat conduction within the metal, but I get totally lost about there. |
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[monojohnny] No, it would keep spinning and being heated while it travels. And yes, //pointing it where you want to go// is totally wrong too. You're already in orbit, so you would point it either forward or backward in your existing orbit in order to raise or lower your orbital velocity and move gradually inwards or outwards from the body you're orbiting, to reach your desired orbit. |
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I was under the impression that those little spinny things don't actually work as pure solar sails, despite common belief. That there is a heat reaction on one side with the small amount of gas remaining in the chamber. Oh, which is what [zen_tom] linked. |
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I don't get the combination of spinning and pointing here. |
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Which axis is this spinning round in relation to its 'exhaust' port ? And in relation to which 'face' is pointing at the sun? |
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It is spinning around the axis through the exhaust port and the "nose" end. It keeps the exhaust port and the nose pointed in one direction, and rotates around that so that it gets evenly heated, like a pig on a spit. |
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Black (to IR wavelengths) is both the best absorber and the best radiator of heat. |
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I'm not trying to magically make photons emit more from one surface than the other. I'm using geometry to attempt to sum up all the radiative vectors from a shape, such that the sum has a non-zero component in one direction. |
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I don't know if this is possible, for two reasons; firstly, I haven't sketched or attempted to calculate the sum of the vectors from a perfectly evenly radiating body of a given shape, and secondly I suspect there might be a balancing mechanism with the heat transfer within the metal that evens things out. |
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/I'm not trying to magically make photons emit more from one surface than the other/ |
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Sure you are. And its not unreasonable. Emissivity varies for different surfaces,and as the first idea pointed out, [Vernon]'s IKECE could be used as a propellant in this context. I wish someone would do the IKECE experiments. |
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Of course a solar sail is more efficient, but as you point out there is no way to tack. This is like having a fan mounted on your sailboat. |
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Hmm. Last night I pondered a long black heatpipe [link] in a silvered
concentrating trough, collecting
side facing the sun, and a [yes]
parabolic bell silvered on the
inside with a black ball at the "cold" end of the heatpipe at its focus. |
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I'm still unsure whether this works or doesn't work, so it's hereby [marked-for-engineering] see [link] |
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Net thrust from all sides of any radiating shape will be zero: assuming the shape, whatever it is, is of a uniform temperature, it will radiate evenly in all directions. |
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Now, if you want to generate more thrust from this, you put it at the center of a large, focused solar sail. You get thrust from the sail, plus thrust from the radiator. The radiator is still generating net zero thrust, but most of the photons it generates in the wrong direction impinge on the solar sail, thrusting it in the correct direction. In this way, you get to increase the ouput of your solar sail. |
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Using thermal shielding to place some of the radiator in shadow would allow it to serve as the receiver for a solar thermal power generation plant, even at distances beyond what are considered practical for solar power. |
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SCIAM, or POPSCI? I seem to remember the latter, but I could be wrong. |
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Actually I think it is New Scientist that you are thinking of(google "New Scientist" "reactionless drive"). Which is a popular science magazine with a very large amount of on line material. |
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They are not exactly rigorous. I have actually corrected them on a couple of things by email and seen them change the article in response (and send me a thank you note). |
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They do tend to publish "Science By Press Release" kind of stuff sometimes. |
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I like this idea, but I'm going to have to do
some more research. Thanks [bunsen] for
making me think. |
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Do bodies radiate the photons normal
to
the emitting surface? If so, these could
be
used to shoot photons at solar sails
from
'stationary' positions in space. Sort of
like
those hot wheels booster wheels. |
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*edit* if we're talking about black body
radiation here, then it will radiate with
equal intensity in all directions. A black
body is a theoretically perfect
Lambertian radiator. See links. |
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eligh: //Net thrust// But the whole thing is not of uniform temperature. There's a hot end (in the parabolic trough facing the sun) and a cold end (the black radiator body in the drive chamber) |
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The drive chamber is basically a heat (IR) torch. If it's pointed into the cold, dark, 3K inky blackness of space, and the other end is pointed at the sun, it must radiate, no? |
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And even in the simple case of a single skinned (tube/ellipsoid/spheroid/bell etc), is there not still a delta_t between the sun-warmed side, and the deep-space facing side? |
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//solar sail// If you take your solar sail, and move it closer towards the radiative body, shrinking it as you go, and wrapping it around into a paraboloid bell, isn't this pretty much what I'm proposing? |
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// There's a hot end / and a cold end // |
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I realise this isn't in the original post, but in a brief anno above. However this is what my vision of the drive has evolved into as I've thought about it. I'll edit the original post to suit. |
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// If it's pointed into the cold, dark, 3K inky blackness of space, and the other end is pointed at the sun, it must radiate, no?// |
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While this was inspired by your CBR, I dont think that has anything to do with the radiation. Once the object reaches the temperature which it will radiate photons, it will do so regardless of temperature differenitial. |
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I like the use of these as boosting stations for solar sail powered ships rather than built into the ships. I envision large mirrors focousing an enormous amount of sunlight onto a blackbody within an eliptical reflector who's focal point is a calculated and optimized distance away. There could be several of these to maintain propulsion, as you get farther away from the sun, you pass into the path of another emitter and reflector pair, furthering your thrust. Each successive photon emmitter would have to have a larger mirror array to collect the sunlight, but the radiator and focousing reflector could be the same size. I'll see about getting a drawing up later (but I make no promise, I have a test this week. Midterms Suck) |
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If you had the parabolic side facing the sun as a blackbody (e.g. carbon) or metal, and the other side shelled with a dissipative ceramic with a very low thermal conductivity (e.g. TPS tiles), then this would work, no problem. |
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[bleh] that idea came up in Solar Spacecraft Propulsion too - the idea right above this one in the index. Problem I see is that each of your collector mirrors would also act as a solar sail and have difficulty keeping station. Although according to the wikipedia Solar sail entry, this effect can actually be used to keep satellites on station in otherwise unstable orbits. |
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